Microencapsulation of cells in semi-permeable membranes has been proposed to prevent their immune destruction following transplantation, thus alleviating the need to use toxic immunosuppressive drugs. This approach can be used for replacing defective organs, such as insulin-producing cells for the treatment of diabetes, and for delivering natural or synthetic therapeutic molecules for the treatment of numerous diseases.
The field of microencapsulation has been faced with a number of issues hindering proper use in physiological conditions. One such issue is the resistance of microcapsules to both chemical and mechanical degradation. More precisely, microcapsules resistant to both chemical and mechanical degradation are crucial in situations wherein microcapsule delivery has to lead to highly efficient and durable treatments.
Strong microcapsules will obviously increase the durability of the transplant. It is also likely to improve long term biocompatibility of microcapsules, since a strong pericapsular reaction always develops around broken or damaged capsules. Microcapsules that can hardly be destroyed in conditions compatible with life would improve the safety of a number of different transplanted cells such as virus-transfected bioengineered cells, immortalized cells or stem cell-derived cells.
Efforts have been made to improve microcapsule strength. Of particular interest is the formation of complexes between negatively charged polyanions such as alginate and positively-charged polycations such as poly-L-lysine (PLL) to form alginate-poly-L-lysine-alginate (APA) microcapsules. This is the most widely used method to microencapsulate cells. However, one of the major drawbacks of the presently used microcapsules is their limited resistance as well as their limited membrane stability.
In order to improve microcapsule strength, the applicants, as well as others, have evaluated the effect on microcapsule strength of modulating intrinsic parameters such as PLL molecular weight, concentration and incubation time and the manuronic acid/guluronic acid ratio of alginate. The formation of neutral capsules by the introduction of a new coating agent has also been investigated. Following these experiments, electrostatic binding between PLL and alginate has been obtained. Nevertheless, poly-L-lysine still competed with other positively charged molecules present in the environment to bind to the alginate beads. A prolonged incubation in solutions with high concentrations of calcium, for example, has shown a displacement of the alginate-poly-L-lysine bonds. In addition, it has been observed that the external sheet of alginate was progressively lost from microcapsules within days or weeks.
To prevent this competition with charged molecules in the environment, a new concept is proposed: the introduction of covalent links into the membrane of the microcapsule. Covalent links are known to be more stable than electrostatic interactions. The challenge is that most methods to create or break a covalent link are incompatible with cell survival.
Introducing covalent links within the structure of the alginate layer has increased the stability of alginate beads. However, for microcapsules comprised of an alginate bead core subsequently coated, the covalent links would only solidify the inside of the microcapsules with no effect on the stability of the semi-permeable membrane or of the outer coats.
In 2001, the Wang group has also suggested a way to create covalent links but this time, within a semi-permeable layer, made of modified poly(allylamine), which plays a role similar to the one played by poly-L-lysine in alginate poly-L-lysine microcapsules (Chang, S. J., et al., Biocompatible microcapsules with enhanced mechanical strength. J Biomed Mater Res 59(1): p. 118–126, 2002; Lu, M. Z., et al., A novel cell encapsulation method using photosensitive poly(allylamine alpha-cyanocinnamylideneacetate. J Microencapsul 17(2): p. 245–251, 2000; and Lu, M. Z., et al., Cell encapsulation with alginate and alpha-phenoxycinnamylidene-acetylated poly(allylamine). Biotechnol Bioeng 70(5): p. 479–483, 2000. To enhance the microcapsule's resistance, Wang et al. proposed to graft a photodimerizable reactive group on the polycationic polymer forming the semi-permeable membrane of microcapsules. This functional reactive group has the particularity to dimerize when exposed to light allowing these cationic polymers to form covalent bonds between one another. However, the remaining problem was that the microcapsule layers were linked to one another by only weak electrostatic bonds.
While the microcapsules known in the art have resulted in the expansion of the present field, there is still a need for a new semi-permeable microcapsule, such as one that exhibits an increased resistance to mechanical stresses as well as an increased resistance to chemical degradation, while permanently preserving a defined molecular cut-off of the semi-permeable membrane.